C equivalent to fstream's peek

Question

I know in C++, you're able to peek at the next character by using: in.peek();.

How would I go about this when trying to "peek" at the next character of a file in C?

ephemient · Accepted Answer · 2010-01-17T22:13:11.410

59

fgetc+ungetc. Maybe something like this:

int fpeek(FILE *stream)
{
    int c;

    c = fgetc(stream);
    ungetc(c, stream);

    return c;
}

edited Jan 17 '10 at 22:13

answered Jan 17 '10 at 21:39

ephemient

3

the conditional is unnecessary: `ungetc(EOF, foo)` is well-defined ("If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged") – Christoph Jan 17 '10 at 21:45
@Christoph: That's handy. My man page didn't include that tidbit, but the one I linked to does... – ephemient Jan 17 '10 at 22:13
@emil: In C, `EOF` may be any negative integer. – dreamlax Jan 17 '10 at 23:05
@dreamlax: I understand that, but you have to represent the negative integer in some way, in binary -1 as a 8 bit character is: 0xff = 11111111. So if getc would return a 8 bit character (0x00-0xff) there wouldn't be enough room for EOF. And as it is a integer let say it is 4 characters 'a 32 bit, then any other value can be returned in the range 0x00000100-0xffffffff I.e: (-2147483648 - -1 and 256 - 2147483647) – emil Jan 18 '10 at 10:25

score 9 · Answer 2 · answered Jan 17 '10 at 21:38

9

You could use a getc followed by an ungetc

answered Jan 17 '10 at 21:38

moonshadow

score 2 · Answer 3 · edited Sep 25 '16 at 12:52

2

you'll need to implement it yourself. use fread to read the next character and fseek to go back to where you were before the read

EDIT:

 int fsneaky(FILE *stream, int8_t *pBuff, int sz) {
    sz = fread(pBuff, 1, sz, stream)
    fseek(pFile, -sz, SEEK_CUR);
    return(sz);
 }

edited Sep 25 '16 at 12:52

nimig18

answered Jan 17 '10 at 21:37

Charles Ma

This won't work if the `FILE*` is unseekable - for example, a terminal or a pipe. – Maya Sep 10 '19 at 18:38

3 Answers3